About Insulations


Electric Insulation System
Insulation Class

Electric insulation system has to withstand variety of stresses during the operation of electrical machines these stresses must be taken into consideration before selecting the correct insulating materials.

1. Mechanical Stress
It arises due to moving parts in the electrical machines, centrifugal forces, magnetic forces, vibration and as a result of fluctuating temperature in winding with changes in load & differential expansion due to difference in thermal coefficient of components giving rise to abrasion.

2. Electrical Stress
It comes in to play due to high voltage gradient, electrical field generated, voltage surges, phase to ground faults, corona discharges, partial discharges, arcing with ground or between phases & harmonics.

3. Thermal Stress
This is produced because of losses in conductor, core losses due to hysteresis & eddy current losses, dielectric heating due to dipole movement, pd activity, harmonic losses and insulations defects. Generally insulating materials are poor conductor of heat which aggravates the problem further.

4. Environmental Stress
Exposure to humidity (moisture), radiation+, oxidation, ozone, salinity chemical fumes, dust & carbon are some of the reasons for environmental stress.

+ Nuclear power plants
^ Refrigeration motor has to withstand CFC attack

Beside above stresses insulation material should withstand the demands of processing arising during the manufacture of electrical machine. Deterioration in insulation system is generally caused by combination of several factors playing simultaneously. While making the choice of a correct insulating material following factors should be considered.

Basic Materials

Electrical insulating materials are characterised by following

1) High dielectric strength.
2) Low dielectric constant.
3) Low dielectric loss (tan delta).
4) High volume and surface resistivity, insulation resistance.
5) High tracking, corona & arc resistance.